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Original question:

LMH5401: Clarification with the datasheet specifications and potential applicability

LMH5401: Overdrive protection

Ash LAW
Prodigy 120 points
Part Number: LMH5401
Other Parts Discussed in Thread: ASH

I am using the LMH5401 as a preamp for a wideband active antenna. We have the system working to our satisfaction. Now I'm trying to figure out a way to protect not just the amp but also downstream circuitry from strong signals that may sometimes be present in our environment. These signals can be very large and potentially damaging. The signals can last for a fairly long time (several 100 milliseconds and have frequencies ranging from 100 MHz to 1 GHZ). Does anyone have an idea of what the best technique might be for this sort of thing.

  • 0
    Guru 140200 points

    Hi Ash,

    is the LMH5401 sitting at the input of signal chain? What is your input signal (frequency, amplitude)? Can you post a schematic?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 120 points

    kai klaas69 said:

    Hi Ash,

    is the LMH5401 sitting at the input of signal chain? What is your input signal (frequency, amplitude)? Can you post a schematic?

    Kai

    Kai,

    Yes, the LMH5401 is sitting at the front of the chain. I can't post the schematic, but right now the way it is the antenna is directly attached to the +ve input of the LMH5401. The other pole is terminated by a resistor that matches the impedance of the antenna. The antenna is a wideband antenna with a constant impedance across the entire frequency range (yes it does, and please lets not get distracted by the antenna). Nominal operating max voltages (aggregated across the entire band) can range from a few hundred microvolts to about about 200 mV. When the interference is present the voltage from the antenna can spike up to 20-40 Volts.

    Thanks

  • 0 in reply to Ash LAW
    Guru 140200 points

    Hi Ash,

    could such a scheme help?

    How much current can the 20...40V spike voltages source?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 120 points

    Kai,

    Something like that might work. I was also considering a zener diode and regular diode type combination. I'm not keen on putting a current limiting resistor in line with the signal because it wrecks the noise floor (we're trying to detect very weak signals) so whatever the antenna pushes out is what we have. I need to get a better understanding of the environment from the folks who know, but I would suspect it could be fairly large.

    Ash

  • 0 in reply to Ash LAW
    Guru 48395 points

    be sure to include any protection device capacitance in your simulations - will typically impair flatness.